Natural avalanches to size 3 were reported Monday, this kind of activity will likley continue into Tuesday. Slopes which have not naturaled are primed for large human triggered avalanches Tuesday.
Confidence
Fair - Freezing levels are uncertain on Wednesday
Weather Forecast
The Purcells have received around 30cm of snow on the storm boards in the North & 20 - 30cm in the South of the region. Snow is expected to continue through the day Monday with a daytime total of 12cm. The region should see another 5cm Monday night, bringing the storm total to aprox. 50cm. Tuesday looks to be clear and cold as freezing levels return to the surface. Winds should be L out of the N/NE @ 2000m Tuesday. Temperatures warm Wednesday as warm air floods the region, we expect a freezing level around 1700 on Wednesday. The warming continues Thursday, the freezing level starts near 1000m, the current model runs show the precip climbing to 2400m in the afternoon.
Avalanche Summary
We received reports late in the day that indicate a large natural avalanche cycle was in progress Monday. A sledder triggered a size 3 avalanche in the Quartz Creek area and while the machine is reportedly totaled, the rider is okay. Control work in the Dogtooth's Sunday produced avalanches to size 2.5. The buried surface hoar continued to preform, numerous soft slab avalanches were remoted by skiers to size 2. One operation in the Northern Purcells reported an avalanche cycle involving wet snow at low elevations which produced avalanches to size 3. In the southern portion of the region Natural and sled triggered avalanches to size 2.5 were reported. With all the new snow & wind I suspect there was widespread natural activity Monday. Visibility was limited, so, we probably won't know the extent of the action until skies clear Tuesday.There are two great observations from professionals working in the region recently. The first one can be found on the MCR, it talks about the widespread & sensitive nature of the SH in the Northern Purcells. You can start here and work your way into the March observations: https://www.acmg.ca/mcr/archives.asp The other is in the form of a youtube video which also deals with the surface hoar, highlighting the potential for remote triggering and large propagations when this layer fails. Check out the shooting cracks in the video. Wild!: https://www.youtube.com/watch?v=UmSJkS3SSbA&feature=youtu.be One of our field tech's checked in after a personal hut trip in the Northern Purcells. He reports a touchy surface hoar buried down 40cm below the snow surface (before the Sun/Mon. storm) which is consistent with observations from the region.
Snowpack Summary
30 cm of storm snow fell across the region during daylight hours Monday accompanied by winds out of the W/NW & W/SW at speeds capable of wind slab formation. Another 15cm are expected before skies clear Tuesday. A cohesive slab continues to settle over the mid-February surface hoar, which is now down 70 to 100 centimeters. Recent reports include whumpfing and cracking, and moderate but sudden compression test results. This persistent weakness is susceptible to remote triggering and has the ability to propagate in low angled terrain, and the overlying slab structure creates the potential for step-down avalanches. Basal facets have only been reactive on steep, shallow, and rocky slopes, but operators continue to monitor this layer in tests. Triggering this deep persistent weak layer is unlikely, but shallow snowpack areas or shallow weak areas adjacent to deeper wind loaded slopes are the most suspect locations.
Problems
Persistent Slabs
Persistent Slab avalanches are the release of a cohesive layer of snow (a slab) in the middle to upper snowpack, when the bond to an underlying persistent weak layer breaks. Persistent layers include: surface hoar, depth hoar, near-surface facets, or faceted snow. Persistent weak layers can continue to produce avalanches for days, weeks or even months, making them especially dangerous and tricky. As additional snow and wind events build a thicker slab on top of the persistent weak layer, this avalanche problem may develop into a Deep Persistent Slab.
Storm Slabs
Storm Slab avalanches are the release of a cohesive layer (a slab) of new snow that breaks within new snow or on the old snow surface. Storm-slabs typically last between a few hours and few days (following snowfall). Storm-slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.
Wind Slabs
Wind Slab avalanches are the release of a cohesive layer of snow (a slab) formed by the wind. Wind typically transports snow from the upwind sides of terrain features and deposits snow on the downwind side. Wind slabs are often smooth and rounded and sometimes sound hollow, and can range from soft to hard. Wind slabs that form over a persistent weak layer (surface hoar, depth hoar, or near-surface facets) may be termed Persistent Slabs or may develop into Persistent Slabs.